Weather in Boston in September

Prime time. If you get to pick your dates, September is the one. Highs average 24.3 °C, lows 13.3 °C, and rain falls on 9 days — nothing in the month is working against you.

September is the highest-scoring month of the twelve. The nearest rival is June, and the month at the other end of the year is January.

Rain on 9 days, 3 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

The averages here are honest — no hidden extremes worth planning around. Across the twenty years measured, the hottest September day reached 36.2 °C and the coldest night fell to 5.3 °C, and the wettest single day dropped 81 mm — roughly 88% of the month's rain in one day.

September is moderately variable: 34 mm in the driest year against 218 mm in the wettest, around an average of 92 mm. Roughly two Septembers in three land near that average, and the third does something else entirely.

Wind is light at 1.7 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 75%, high enough to notice on the warm days without dominating them.

How September sits in the year

The month is on the way down: 3.7 °C cooler than August, with October dropping another 7.4 °C. The first week and the last are not the same trip.

Daylight in September

12.6 hours of daylight, losing 1.4 hours across the month. That is between Boston's extremes of 15.3 and 9.1 hours, so days are long enough not to constrain the plan.

Sea temperature

The sea sits at about 17.6 °C in September — cold — a quick dip rather than a swim, and a wetsuit if you are in for longer. This is the warmest the water gets all year.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Boston versus nearby

If Boston is one option among several, the nearby comparison is worth making. New York City is close enough to be a coin toss — within a degree on temperature, the same number of rain days. Bar Harbor is close enough to be a coin toss — 5 °C cooler, 1 day more rain.

So: September is the answer for Boston. If it is booked out or priced out, June is the next best thing at 6 points behind, and January is the month to keep off the shortlist.

Every month in Boston

See all twelve months in Boston, or where else to go in September.